1. Field of the Invention
The present invention relates to improved methods and systems for electrochemically producing electrical power using metal-air fuel cell battery technology.
2. Description of the Prior Art
More than ever, there is a great need in the art for ways and means of reliably producing small and large amounts of electrical energy for powering various types of electrical systems and devices. It can be helpful to classify these various types electrical systems and devices (conventionally called "electrical loads") into four different market areas, namely: the Portable Electronics Market which includes products such as portable computers, cellular phones, camcorders, cassette tape players, etc. requiring less than 100 Watts; the Portable Electric Power Tools Market which includes products such as lawn mowers, screw drivers, drills, saws, etc. requiring more than 100 W but less than 1.0 kiloWatt; the Transportation Market which includes products such as power passenger vehicles, buses, golf carts, motorcycles, boats, etc. requiring more than 1.0 kiloWatt but less than 100 kiloWatt; and the Stationary Power Market which includes products such as multi-megawatt systems for powering homes, schools, factories, office buildings, and other distributed generation applications, requiring more than 10 kiloWatt, but less than 200 kilowatts.
Presently, there exists a number of different concepts and techniques for producing electrical power. Among these various concepts and techniques, the technique of Electrochemical Energy Conversion is most popular in the contemporary period inasmuch as it enables the direct production of electrical power from chemical compositions with relatively high energy density on a weight basis (measured in Watt Hours per Kilogram) with relatively high current densities (measured in Amperes per Centimeters2). Examples of devices based on the Electrochemical Energy Conversion concept include: battery cells; fuel cells; and fuel cell batteries (FCB).
Early storage batteries employed lead-acid cells, and then other combinations such a nickel with iron, cadmium, zinc or hydrogen and silver-iron, zinc-bromine, zinc-chlorine were developed with increasing energy quantities per weight. Presently, conventional battery cells are based on one of the following composition pairs: lead acid; nickel-cadmium; and nickel-metal hydrides (NiMH) As battery developers seek to improve energy storage capacity and energy stored per kilogram, they continue to focus on the use of materials. The development of the zinc-air battery is indicative of this research approach.
The electrochemical storage battery is a well known device having many applications. The storage, or secondary battery, is characterized in being capable of accepting direct-current (DC) electrical energy in a changing phase, retaining the energy in the form of chemical energy in the charge retention phase and releasing its energy on being connected to an external load in the discharge phase. The storage battery is capable of repeatedly performing these three phases over a reasonable life cycle.
The structure of the storage battery is typically a construction including one or more identical units called cells. Each cell contains plates referred to as positive (anode) and negative (cathode) electrodes contained in an electrolyte. When a charged storage battery cell is discharged through a load, the plates and the electrolyte undergo a chemical change wherein the negative cathode loses electrons and the positive anode gains electrons, thereby providing a current flow. During charging, the original conditions of the battery are restored by passing through it a current opposite to that produced during the discharge.
Conventional battery technologies based on lead acid, nickel-cadmium, or nickel-metal hydrides have limited operation time, long recharge time, low energy density, hazardous chemical materials requiring special encapsulation containers and careful disposal, fixed electrode areas, and in electrical automobile applications, conventional battery systems result in limited driving distances. Nickel-metal hydride (NiMH) batteries are attractive insofar that they eliminate cadmium, a very toxic substance; however, they deliver less power, have a faster self-discharge rate, and are less tolerant of overcharging.
Because of the enormous market potential, the shortcomings of conventional batteries represent a great opportunity to innovators and entrepreneurs to introduce battery products based on radically new concepts which overcome those shortcomings. Indeed, there is ample evidence of intense R&D activities and significant investments directed towards battery development.
Lithium-polymer batteries promise substantial improvements in energy density. Lithium battery systems employ a lithium anode, a polymer electrolyte and a composite cathode such as CuO, CuS, or FeS. Battery cells of this type are described in the Aug. 19, 1991, Electronic Engineering Times in the publication "Batteries Slim Down For Portability" by Colin Mackay and Robert Kline, Jr. at page 52. However, a major drawback with this battery cell design is that lithium's high reactivity with liquid electrolytes erodes the electrodes of such battery cells. While recent developments in solid state electrolytes have reduced this problem, a number of problems still remain, namely: dendrite formation on the electrodes; and the hazardous effects of lithium on the environmental.
Zinc-air battery technology is environmental friendly, but current batteries are limited to fixed area, resulting in low perceived specific power rating.
Batteries which are reliable, environmentally benign, have energy densities in excess of 200 Watt-hour per kilograms (lead-acid has only 35), and can be recharged faster, can find uses not only in portable electronics but also in consumer appliances, electric vehicles, and in the utilities industry; however, batteries with all of these desirable characteristics do not yet exist.
All conventional batteries, including conventional zinc-air batteries, are designed to have a fixed area, and thus a fixed stored energy determined by the voltage times the charge per unit volume. Traditional battery designers continue to adopt the fixed area design methodology and, therefore, are hindered by fundamental constraints including: (1) the larger the battery capacity, the longer it takes to recharge; (2) every unit weight of the anode is nearly matched by the weight of the cathode, the weight of the electrolyte, as well as the weight of the container; this overhead is the source of low energy density; (3) pulse power is inversely related to the energy capacity; and (4) only one set of electrodes are available for the sequential discharge and recharge cycles.
Fuel Cells have been known for more than one hundred years. Conventional fuel cells are electrochemical devices that convert chemical energy of the fuel directly into usable electricity and heat without combustion of the fuel. The electrochemical reactions are not reversible (i.e. rechargeable). Fuel cells are similar to battery cells in that both produce a DC current by using an electrochemical process. Both fuel cells and batteries have positive and negative electrodes (i.e. the anodes and cathodes) and an ionic conductor or electrolyte. The primary difference between fuel cells and battery cells is that battery cells have only a limited amount of stored energy whereas fuel cells will continue to produce electrical power output as long as fuel and oxidant are supplied thereto.
Conventional fuel cells operate by combining hydrogen with oxygen to release electricity (i.e. charge), heat, and water. The supply of fuel can be pure hydrogen. Hydrogen can also be extracted from natural gas, or other hydrocarbons by using a reformer. Fuel cells emit essentially none of the sulfur and nitrogen compounds released by conventional "combustion-based" electrical power generating methods employing fossil and like kinds of fuel.
Presently, several different conventional fuel cell technologies are being considered by the power industry for power generation. Phosphoric Acid Fuel Cells (PAFCs); Molten Carbonate Fuel Cells (MCFCs); Solid Oxide Fuel Cells (SOFCs); and Solid Polymer Fuel Cells (SPFCs). These different fuel cell technologies differ in terms of the composition of the electrolyte used, and each are presently at different stages of development.
Phosphoric Acid Fuel Cells (PAFCs) are the maturest of the fuel cell technologies. Platinum is required as a catalyst for the electrodes. "Reforming" of the natural gas feedstock to a hydrogen-rich gas must be accomplished outside the fuel cell stacks. Due to PAFC materials and complexity, capital costs are higher and efficiencies are lower than those projected for the Molten Carbonate Fuel Cells and Solid Oxide Fuel Cells.
Molten Carbonate Fuel Cells (MCFCs) operate at higher temperatures, (at or slightly above ambient pressure) and uses less expensive nickel-based electrodes than PAFCs. Reforming can occur either inside or outside the fuel cell stacks. The higher operating temperature of MCFCs provides the opportunity for achieving higher overall system efficiencies (potential for heat rates below 7,500 Btu/kWh) and greater flexibility in the use of fuels. On the other hand, the higher operating temperature places severe corrosion and sealing demands on the stability and life of cell components, particularly in the aggressive environment of the molten carbonate electrolyte.
Solid Oxide Fuel Cells (SOFCs) are the least mature of fuel cell technologies. SOFCs use a zirconia ceramic as the electrolyte. The electrochemical conversion process occurs at very high temperatures (1200(C) and thus supports internal reforming. The cells may be either flat plates or tubular cylinders. There are basic manufacturing challenges, as yet unsolved, in mass producing these cells. SOFCs promise to operate at moderately high efficiencies with a high-grade waste heat output.
Solid Polymer Fuel Cells (also known as the proton exchange membrane fuel cell, PEMFC) operates at a much lower temperature (around 90.degree. C.), because of the limitations imposed by the thermal properties of the membrane materials. SPFCs are quickly contaminated by CO. They require cooling and management of the exhaust water in order to function properly. The main focus of current designs is transportation applications, as there are advantages to having a solid electrolyte for safety. The heat produced by this type of fuel cell is not adequate for any significant by-product usage. There is a possibility that this particular type of fuel cell can be used for both transportation and stationary power applications.
Prior art hydrogen-oxygen fuel cells of the type described above suffer from a number of shortcomings and drawbacks that have restricted their widespread usage. In particular, prior art hydrogen-oxygen fuel cells require operation at either high pressure and/or temperature. The hydrogen-oxygen fuel poses risk of explosion and requires careful handling and distribution. These fuel cells require a co-generation application for the heat produced in order to reach high efficiency levels.
These fuel cells are unlikely to be scaled down for use in portable electronic applications. The expected cost per kW for these fuel cell power generation systems is still far above the target of $1,000/kW. Gradual stack degradation over their projected life mandates costly periodic replacement of the cell stacks.
U.S. Pat. No. 3,432,354 to Jost, apparently a pioneering U.S. Patent, discloses an air-metal fuel cell battery (FCB) system, in which the anode is moved past the stationary cathode during discharge and charging operations. In the illustrative embodiments disclosed therein, the anode is based on metals such as zinc, aluminum, and other alloys, and not on hydrogen. The anode material is arranged as a roll of thin zinc foil wound on a supply roller. As the fuel moves past a discharge cathode, and is taken up on a take-up roller in the presence of an electrolyte, electrical power is produced across the anode and cathode and removed by an electrical load connected thereto. In this FCB system design, the anode tape is required to be pervious to the electrolyte used. Moreover, the arrangement of the anode and cathode elements render it impossible to produce high energy density systems.
U.S. Pat. No. 5,250,370 to Applicant discloses an improved air-metal FCB system, wherein the ratio of the recharge cathode area to the discharge cathode area is much larger than unity, resulting in a much faster recharge time.
Notwithstanding such limitations, the air-metal FCB design of the type described in U.S. Pat. Nos. 5,250,370 and 3,432,354 has numerous advantages over traditional hydrogen-based fuel cells.
In particular, the supply of energy provided from air-metal FCB systems is inexhaustible because the fuel, zinc, is plentiful and can exist either as the metal or its oxide but will never vanish from the earth. Plentiful solar, hydroelectric, or other forms of energy can be used to convert the zinc from its oxide product back to the metallic fuel form.
The air-metal FCB system has the highest energy density of any alternative technology, i.e. in excess of 500 Watt Hour/Kg. The air-metal FCB allows more than 24 hours of continuous operation for either notebook computer or cellular phone, assuming batteries whose dimensions are comparable to existing ones. When used in electric vehicles (EVs), the FCB has a range of about 500 miles.
Unlike conventional hydrogen-oxygen fuel cells which require refilling, the fuel of an air-metal FCB is recoverable by electrically recharging. The fuel of the air-metal FCB is solid state, therefore, it is safe and easy to handle and store. In contrast to hydrogen-oxygen FCBs, which use methane, natural gas or LNG to provide sources of hydrogen, and emit polluting gases, the air-metal FCB results in zero emission. The air-metal FCB operates at ambient temperature, whereas hydrogen-oxygen fuel cells operate at temperatures in the range of 150.degree. C. to 1000.degree. C. Air-metal FCBs are capable of delivering higher output voltages (1.5-3 Volts) than conventional fuel cells (&lt;0.8V).
The air-metal FCB has two simultaneous functions, energy storage (rechargeable secondary battery functions) and energy generation (fuel cell function). Consequently, the air-metal FCB can charge and recharge simultaneously and automatically adapt to different load conditions while maintaining peak efficiency. The output power density of the air-metal FCB is independent of its energy storage capacity.
While air-metal FCB technology offers fundamental advantages of FCB systems over conventional fuel cell systems, prior art air-metal FCB systems suffer from a number of shortcomings and drawbacks.
In particular, the output voltages produced from prior art air-metal FCB systems are fixed by virtue of the construction of the cathode and anode structures of such prior art systems.
In prior art air-metal FCB systems, the physical configuration of the metal (e.g. Zinc) fuel in relation to the air-pervious cathode structure has not enabled the design or manufacture of electrochemical power supplies with ultra-compact construction required for portable electronic devices, such as radios, cellular-phones, laptop computers, and the like.
In prior art air-metal FCB systems, the mechanisms used to supply metal fuel to the air-pervious cathode structure have been unsuitable for meeting the electrical energy requirements of various classes of users. For example, in connection with low power consuming devices, such as cellular phones and laptop computers, it has not been possible to design or make air-metal FCB systems of ultra-compact design.
Thus, there is a great need in the art for an improved method and apparatus for electrochemically producing electrical power while overcoming all of the limitations of known battery and conventional fuel cell technologies.